1. Field of the Invention
The present invention relates to a method of manufacturing potassium niobate single crystal thin films, and to surface acoustic wave elements, frequency filters, frequency oscillators, electronic circuits, and electronic apparatuses, each having the potassium niobate single crystal thin film.
2. Description of Related Art
There has been a remarkable expansion in the demand for surface acoustic wave elements with rapid developments in telecommunications centered on mobile communication, which is typified by mobile telephones. Some trends in the development of surface acoustic wave elements include size reduction, increasing efficiency, and increasingly higher frequencies, as in mobile telephones. In order to attain these, a larger electromechanical coupling coefficient (k2 hereinbelow) and a higher surface acoustic wave propagation velocity are necessary. For example, when used as a high frequency filter, a high k2 is desirable in order to obtain a small loss and a wide bandwidth. In order to make the resonance frequency higher, a material having a higher acoustic velocity is desirable in view of the limits of the design rules for the pitch of inter-digital transducers (IDT hereinbelow). Furthermore, in order to stabilize the characteristics of the temperature range in which surface acoustic wave elements are used, the center frequency temperature coefficient (TCF) must be small.
Conventionally, surface acoustic wave elements generally have a structure in which an IDT is formed on a single crystal piezoelectric body. Representative piezoelectric single crystals are those of quartz, lithium niobate (LiNbO3 hereinbelow), or lithium tantalate (LiTaO3 hereinbelow). For example, in an RF filter requiring a broad band and low loss in the pass band, LiNbO3, which has a large k2, is used. In contrast, in an IF filter requiring stable temperature characteristics even in a narrow band, quartz, which has a small TCF, is used. Furthermore, LiTaO3 plays an intermediate role because its k2 and TCF are each between those of LiNbO3 and quartz. However, even for LiTaO3, which has the highest k2, the k2 is about 20%.
Recently, a cut angle that exhibits a large k2 value has been discovered in potassium niobate (KNbO3 hereinbelow) single crystal (a=0.5695 nm, b=0.5721 nm, c=0.3973 nm; below, the orthorhombic crystal is represented by these indices). As reported in Electron. Lett. Vol. 33 (1997) 193, it can be predicted by calculation that a 0° Y-cut X-propagation (hereinbelow 0° Y-X) KNbO3 single crystal plate shows an extremely high value of k2=53%. Furthermore, as reported in Jpn. J. Appl. Phys. Vol. 37 (1998) 2929, it has been experimentally confirmed that a 0° Y-X KNbO3 single crystal plate demonstrates a high value of k2 (about 50%), and it is reported that the oscillation frequency of the filter using the Y-X KNbO3 single crystal plate rotated from 45° to 75° demonstrates zero temperature properties at room temperature. Published Japanese Patent Application No. Hei 10-65488 discloses that the single crystal plates are used as a surface acoustic wave substrate.
In surface acoustic wave elements that use a piezoelectric single crystal substrate, characteristics such as the k2, temperature coefficient, and sound velocity are values intrinsic to the material, and are determined by the cut angle and the propagation direction. A 0° Y-X KNbO3 single crystal substrate has a superior k2, but the zero temperature properties like those of the Y-X KNbO3 single crystal substrate rotated from 45° to 75° are not exhibited at room temperature. In addition, the propagation speed is low in comparison to that of strontium titanate (SrTiO3 hereinbelow) and calcium titanate (CaTiO3 hereinbelow), which are also perovskite-type oxides. Thus, when only a KNbO3 single crystal substrate is used, the sound velocity, high k2, and zero temperature properties cannot all be satisfied.
Thus, a piezoelectric thin film is laminated on some type of substrate, film thickness is controlled, and it is thereby anticipated that the sound velocity, k2, and temperature characteristics will be improved. Examples include a zinc oxide (ZnO hereinbelow) thin film formed on a sapphire substrate, as reported in Jpn. J. Appl. Phys. Vol 32 (1993) 2337, or a LiNbO3 thin film formed on a sapphire substrate, as reported in Jpn. J. Appl. Phys. Vol. 32 (1993) L745. Therefore, for KNbO3 as well, it is anticipated that all properties will be improved by depositing a thin film onto a substrate.
It is preferable that the piezoelectric thin film be oriented in an optimal direction in order to exhibit its k2 and temperature characteristics, and that it be a flat, compact epitaxial film in order to minimize as much as possible the loss that accompanies leaky wave propagation. A Y-X KNbO3 thin film having a k2 of about 50% corresponds to the pseudo-cubic crystal (100), and the 90° Y-X KNbO3 thin film having a k2 of 10% corresponds to the pseudo-cubic crystal (110). Therefore, for example, by using a SrTiO3 (100) or (110) single crystal substrate, it is possible to obtain a Y-X KNbO3 thin film having a k2 of about 50% or a 90° Y-X KNbO3 thin film having a k2 of about 10%.
In manufacturing KNbO3 thin films according to a typical method of manufacturing thin films, such as a conventional vapor deposition or sol-gel method, since the saturation vapor pressure of K is extremely high compared to that of Nb, K vaporizes easily in comparison to Nb, which biases the composition of the thin film manufactured towards excess Nb compared with the initial composition. In order to compensate for this alteration in composition, according to Appl. Phys. Lett, Vol. 68 (1996) 1488, a target made to have excess K is used.
However, as is clear from the phase diagram of the K2O—Nb2O5 shown in FIG. 19 cited from J. Am. Chem. Soc. Vol. 77 (1955) 2117, on the side where the composition of the KNbO3 has K in excess, 3K2O.Nb2O5 compound is present. Below the eutectic temperature of 845° C. for KNbO3 and 3K2O.Nb2O5, both KNbO3 and 3K2O.Nb2O5 coexist as a solid phase. On the side where the composition of the KNbO3 has Nb in excess, 2K2O.3Nb2O5 compound is present. Below the melting point of 1039° C. of KNbO3, both KNbO3 and 2K2O.3Nb2O5 coexist in a solid phase. Therefore, in manufacturing by a vapor deposition method, when the initial material ablated by a laser beam has arrived at the substrate, if the composition ratio is not exactly K:Nb=50:50 and is shifted either to the K excess side or the Nb excess side, the thin film manufactured includes a different phase, and s single phase is not produced.
On the other hand, in the case of KNbO3 bulk single crystal, according to J. Crystal Growth Vol. 78 (1986) 431, using a Top-Seeded Solution Growth (TSSG) method, a large single crystal can be obtained, by pulling up the single seed crystal from a liquid phase that has K in slight excess over K:Nb=50:50. In the K2O—Nb2O5 two dimensional phase diagram of FIG. 19, this may be obtained by placing the starting material, having a composition ratio from K2O:Nb2O5=50:50 to about K2O:Nb2O5=65:35, into a coexistent region for the liquid phase and the KNbO3 that exists in an area above the eutectic temperature of 845° C. for KNbO3 and 3K2O.Nb2O5. That is, in FIG. 2, when the starting material having composition C1 is cooled from the liquidus temperature T1 to the crystal growth temperature T2, the KNbO3 is precipitated from the liquid phase and the liquid phase shifts to composition C2 on the K excess side, in which T2 serves as the liquidus temperature. Because the crystal growth rate in this case becomes higher as C1-C2 becomes larger, a composition having slightly more K than KNbO3 and close to KNbO3 is cooled down to a region in the vicinity of the eutectic temperature 845° C. of KNbO3 and 3K2O.Nb2O5. The above behavior occurs in the air, and furthermore, occurs when a KNbO3 bulk single crystal is grown from a high volume liquid phase.
On the other hand, there have been developed some processes for applying a crystal growth process, by which a single crystal is precipitated from a liquid phase in the air by a TSSG method, to a thin film manufacturing process by a evaporation deposition method at a reduced pressure. One of the processes is a tri-phase-epitaxy method, which stacks a gas phase material on a substrate that is held at a temperature in the solid-liquid coexisting region and precipitates the solid phase from the liquid phase. In a material of NdBa2Cu3Ox, after a single crystal thin film is grown, only the residue of the liquid phase BaCuO2.CuO are selectively etched to produce a single crystal thin film. This is explained in Appl. Phys. Lett. Vol. 80 (2002) 61.
However, the simple application of the tri-phase-epitaxy method to the manufacturing of a KNbO3 single crystal thin film did not allow selective etching of the residue of the liquid phase 3K2O.Nb2O5 after a single crystal thin film was grown. Accordingly, since the liquid phase remained on the surface of the single crystal, a thin film having an excellent surface morphology could not be obtained.
There was a need in this situation for this invention to be made. The invention provides a method of manufacturing KNbO3 single crystal thin film having an excellent surface morphology and a high quality single phase. In addition, using the thin film produced by the method, the invention provides a surface acoustic wave element that has high k2, and is broadband advantageous in downsizing and saving power, a frequency filter, a frequency oscillator, an electronic circuit, and an electronic device.